The present invention generally relates to fluid pumps and motors. The invention particularly relates to piston and cylinder assemblies suitable for use in positive displacement machines.
Axial piston machines are a type of positive displacement machine and generally comprise an array of cylindrical-shaped pistons that reciprocate within cylindrical bores within a cylinder block. In typical axial piston machines, the piston-cylinder combinations are parallel and arranged in a circular array within the cylinder block. An inlet/outlet port is defined at one end the cylinder block for each individual piston-cylinder combination, such that a working fluid can be drawn into and expelled from each cylinder bore through the port as the piston within the cylinder bore is reciprocated. The end of the cylinder block containing the inlet/outlet ports defines an axial sliding bearing surface that abuts a surface of a valve plate, while the opposite end of the cylinder block is connected to a drive shaft for rotation of the cylinder block. The valve plate defines an inlet opening and an outlet opening that are sequentially aligned with the inlet/outlet of each cylinder bore, so that the working fluid is drawn into each cylinder bore through the cylinder bore's inlet/outlet port when aligned with the valve plate inlet opening and expelled from each cylinder bore through the cylinder bore's inlet/outlet port when aligned with the valve plate outlet opening.
One end of each piston is in contact, either directly or through one or more intermediate components (for example, an attached slipper), with a swash plate inclined relative to the axis of the cylinder block. Generally, the swash plate may remain stationary while the cylinder block rotates, or the swash plate rotates while the cylinder block remains stationary, in order to produce axial motion in the pistons. The stroke length of each piston, and therefore displacement of the piston-cylinder combinations, can be made variable by changing the inclination (cam angle) of the swash plate. To provide this capability, the protruding end of each piston may be configured to have a ball-and-socket arrangement. The socket portion of this arrangement may be a slipper may have a planar surface that bears against the swash plate.
Between each piston and the wall of the cylinder bore in which it is received, there exists what will be referred to herein as a piston-cylinder lubrication interface. Within this interface, the bore and piston have opposing bearing surfaces with a diametrical clearance therebetween that defines a lubrication gap between the piston and bore wall. Within this lubrication gap, a continuous film of the working fluid is preferably always present to provide a bearing function that prevents direct contact between the piston and bore wall. Conventional axial piston machines lack sealing elements between their pistons and cylinder bore walls, and therefore the fluid film within the lubrication gap also serves as a hydrodynamic seal to minimize fluid leakage between the piston and the bore wall. Consequently, the sliding bearing surfaces of the piston and cylinder bore wall have both a load-bearing function and a sealing function, which differentiates piston-cylinder sliding bearings of axial piston machines from typical bearing applications that have only a load-bearing function.
Hydraulic fluids are ordinarily used to operate axial piston machines at high pressures, for example, operating pressures of about 300 to 420 bar. Pistons of swash plate type axial piston machines are often subjected to a significant dynamically changing side load during operation due to the combination of these high operating pressures and the variable cam angle of the swash plate. As a result of this off-axis eccentric loading, the lubrication gap between the piston and bore within the piston-cylinder lubrication interface varies along the length of the piston.
Though oil is generally used as the hydraulic working fluid in axial piston machines, the use of water in place of oil would provide several advantages. For example, water's low cost, environmentally friendly properties, thermal conductivity, bulk modulus, resistance to fire, and film strength make it a desirable working fluid relative to oil. However, because water has an extremely low viscosity, its use in axial piston machines is associated with high leakage rates and thus high power losses. More importantly, the low viscosity of water often makes it difficult to build up enough hydrodynamic pressure to perform the required bearing function in the piston-cylinder lubrication interface of swash plate type axial piston units. As noted above, very high side loads are often imposed on the pistons of these units, which increase significantly with increasingly higher operating pressures. As the side load rises, the piston-cylinder lubrication interface provided by water (and other low-viscosity working fluids) has an increased difficulty preventing metal-to-metal contact between the piston and cylinder bore wall, which can lead to catastrophic component failure. For this reason, axial piston machines currently are limited to a maximum operating pressure of approximately 200 bar when using water as the working fluid.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if axial piston machines were available that were capable of operating at pressures above 200 bar, and more preferably 300 bar, while using a low viscosity working fluid, such as water, and yet were capable of eliminating or at least significantly reducing metal-to-metal contact between their pistons and cylinder bore walls.